<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>上中下、左中右</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        .app {
            display: flex;
            flex-direction: column;
            width: 100vw;
            height: 100vh;
        }
        .top {
            height: 100px;
            background-color: #f5dc3a;
        }
        .mid {
            flex: 1;
            background-color: #e15761;
            display: flex;
        }
        .bot {
            height: 100px;
            background-color: #800020;
        }
        .left {
            width: 100px;
            background-color: #FF6D9C;
        }
        .center {
            flex: 1;
            background-color: #56a5f1;
        }
        .right {
            width: 100px;
            background-color: #FC7777;
        }
    </style>
</head>
<body>
    <div class="app">
        <div class="top">Header</div>
        <div class="mid">
            <div class="left">left</div>
            <div class="center">
                <p>头部、底部固定高度是100px，中间内容高度可变</p>
            </div>
            <div class="right">right</div>
        </div>
        <div class="bot">Footer</div>
    </div>
</body>
</html>